In a conventional diagnosis apparatus for a field winding type rotating electric machine, a voltage of a field winding undergoing a fault diagnosis in a field winding type rotating electric machine is maintained at a diagnosis reference potential obtained by dividing a positive electrode voltage of a direct current power supply using a bias resistor, and when a measured voltage of the field winding is determined to have shifted from the diagnosis reference potential to the positive electrode voltage or a negative electrode voltage of the direct current power supply, a power fault, an earth fault, and an open fault are determined to have occurred in a field circuit that includes a field driving circuit and the field winding (see Patent Document 1, for example).